1. Field of the invention
The present invention relates to the casings of turboshaft engines for aircraft, and in particular to the joining means between the individual shells forming the casing.
2. Summary of the prior art
In general, turboshaft engine casings are formed by shells, either circular or semi-circular, which are joined together by means of radially outwardly directed coupling flanges on the shells, the faces of the flanges in contact with one another being provided with evenly spaced holes for the passage of fixing screws or bolts. The thickness of the flanges and the diameter of the screws or bolts are determined with regard to the axial stresses to which the casing is subjected and in accordance with precise standards established for each type of engine.
New types of turboshaft engines have recently appeared, such as turbojet engines with high speed propellers comprising a gas generator driving free counter-rotating turbines which themselves drive counter-rotating propellers of either the pusher or tractor type.
The standards for calculating the strength of the casings of such engines must take into account all eventualities that might be encountered so as to ensure that they are sufficiently robust to counter any risk of accidental destruction of the engine. One of the events with a low probability rating, but which must not be overlooked is the loss or breakage of the propeller blades. Such an exceptional breakage could be caused, for example, by the entry into the propeller doublet of an object emanating from the aircraft (door, piece of tire tread, etc.), either during take off or during flight.
In such a case, the breakage of the propeller blades would impose on the coupling flanges of the casing shells stresses of the order of 50 tonnes. If the thickness of the flanges and the diameters of the fixing bolts have to be of a size to withstand such stresses, this would involve a considerable increase in the weight and radial bulk of the engine, which is incompatible with the traditionally severe requirements for turbojet engines of low overall size and lightness.
It is therefore an object of the present invention to provide a new configuration for casing flanges which enables them to withstand exceptionally large axial loads without increasing the weight or overall size of the engine.